The present invention relates to a drive system. In particular, but not exclusively, the present invention relates to a drilling assembly for a well; a bearing mechanism for a drilling assembly adapted to be located in a well; a sealing assembly for a generally hollow body and a method of sealing therefor; a drilling assembly for a well including a substantially shock eliminating coupling assembly; a substantially shock eliminating coupling assembly for a drilling assembly; and an assembly for location in a hollow body and for transferring a rotational drive force therethrough.
Drilling assemblies such as those used in the drilling of a borehole of an oil or gas well often include drilling motors which form part of a drill string used to drill the borehole. The drilling motor is coupled to a drill bit provided lowermost on the drill string, and which is coupled to the drill bit by a rotatable drive shaft. Typical conventional drilling motors include Positive Displacement Motors (PDMs) and turbines, both of which are fluid driven by a drilling fluid pumped down the drill string from the surface and through the drilling motor. The drilling fluid exits the drill string through ports in the drill bit, to carry drill cuttings from the drill bit and through the borehole to surface. PDMs typically operate at a slow rotational velocity with a high torque output, whilst turbines typically operate at high rotational velocities with a low output torque.
It is normally desired to carry out drilling operations in a low speed, high torque operation, reducing the likelihood of the drill bit sticking and reducing the likelihood of damage in the event that the drill bit does become stuck. PDMs are therefore preferred for low speedy high torque operations, however, PDMs have limitations in that they include elastomeric components, including the PDM stator, and the high pressures and temperatures experienced downhole during a drilling operation often lead to permanent damage of the elastomeric components, which can cause failure of the PDM and require frequent replacement. It is therefore preferred to use turbines as drilling motors which do not usually include elastomeric components. However, as turbines are high speed, low output torque motors, it is required to provide a gear reduction mechanism to reduce the rotational velocity and increase the output torque of the turbine. An alternative to the use of PDMs and turbines is the provision of electric motor drive systems. However, such systems suffer from the disadvantages of requiring electrical power and control connections to surface, which connections are complex and expensive to run and operate and susceptible to damage.
The development of low speed, high torque turbine driven drilling assemblies such as turbodrills has been achieved by the utilisation of a gear reduction mechanism in the drilling assembly. Downhole drilling assemblies have a tubular body outer diameter size limitation determined by the size of the hole to be drilled. Accordingly, the gear reduction mechanisms are typically of the epicyclical type, these being well known and having been developed for downhole applications in particular in the drilling industry of the former Soviet Union, as well as by companies in Canada, Great Britain, Germany and others.
Typically, such drilling assemblies comprise a turbine section consisting of a plurality of turbine power stages including rotors and stators, commonly mounted on a rotating drive shaft and contained within a tubular body. The turbine is connected to a drill bit drive shaft via a gear reduction mechanism including axial bearings, the axial bearings being required to absorb axial hydraulic thrust and mechanical loads. The gear mechanism is normally sealed to retain system lubrication oil and to attempt to prevent the ingress of drilling fluids into the gear mechanism. Typically, the sealed gear mechanism will contain the axial thrust bearings arranged as either a “balanced arrangement”, where drill bit loads act against hydraulic loads (due to the pressure of the drilling fluid), or “non-balanced arrangements”, where the axial thrust bearings for the hydraulic and mechanical loads are separated.
Such arrangements present various problems, including that turbine axial loading and vibration, hydraulic and mechanical bearing loadings, vibration and shock are transmitted directly into the gear reduction mechanism; additional heat and vibration is generated within the gear reduction mechanism; and failure of the sealed gear reduction mechanism (due, for example, to abrasive drilling fluid entering the system) results in axial bearing failure, causing extensive and costly damage to the gear reduction mechanism, the turbine and further damage to the bearings.
Such typical known drilling assemblies are disclosed in U.S. Pat. Nos. 3,365,170 (Whittle), 4,222,445 (Vadetsky et al), 4,329,127 (Tschirky et al), 4,683,964 (Wenzel), United Kingdom Patent Publication No. 2,073,285 (Zahnradfabrik), Canadian Patent No. 1257865 (Dreco) and International Patent Application No. PCT/EP97/06060 (Tiebo Tiefbohrservice GmbH & Co. KG).
U.S. Pat. No. 3,365,170 discloses a turbo-drill with inner and outer contra-rotating turbines. Speed reduction gearing is provided in the turbo-drill as part of the same assembly as a turbine and a bearing assembly for absorbing axial and radial loads exerted on the turbo-drill. A complex arrangement is provided for oil lubrication of the gearing and bearings which includes a pump for supplying oil under pressure to an oil chamber. Thus the bearings are contained within the gear reduction mechanism, the axial bearings in particular having a direct link to the gear mechanism. The lubrication oil provided for the bearings and gear mechanism is at substantially the same pressure as the drilling fluid which powers the turbine and is provided from a common supply. Disadvantages associated with the assembly of U.S. Pat. No. 3,365,170 include that the assembly requires an axial bearing mechanism in the immediate vicinity to the gear reduction mechanism. This may lead to failure as discussed above.
U.S. Pat. No. 4,222,445 discloses a reduction unit for a drilling motor. The casing of the assembly carries a reduction gear in a sealed chamber, with input and output shafts and with roller bearings provided for the shafts. The bearings and reduction gear are disposed in the sealed chamber which contains lubricating oil, and dividing spaces are provided containing a “buffer” fluid to protect the seals from drilling fluid. The input and output shafts carry axial loads directly to the reduction gearing, and the bearings, together with additional separate spherical bearings, are provided in the same unit as the reduction gear. This may lead to failure as discussed above. Furthermore, in the event of leakage of oil from the oil filled chamber, where the internal pressure is maintained substantially constant, drilling mud ingress is accepted, following depletion of the buffer fluid, which initially replaces lost lubrication oil.
U.S. Pat. No. 4,329,127 discloses a bearing assembly for use with a downhole fluid driven motor, and is directed to providing sealing means isolated from drilling fluid. Various radial and thrust bearings are provided in a housing, as well as shock absorbing and bearing loading spring means. The assembly includes a seal which is a complex mechanism including inner and outer reservoirs, one of which carries a material such as grease whilst the other carries a lubricating material such as oil, for lubricating the bearings and other components of the assembly. It is specifically desired that there is substantially no differential pressure across the two reservoirs. Furthermore, it is accepted that there may be drilling mud contamination into the outer grease carrying chamber and it is further accepted that particulate material from the drilling fluid will eventually penetrate through the seal to the bearings and gearing. This may lead to failure of the bearings due to wear by abrasive drilling fluid.
U.S. Pat. No. 4,683,964 discloses an improved downhole drill bit drive apparatus, and particularly relates to an improved sealing arrangement for the bearing assembly of the drive apparatus. A bearing chamber is defined by a casing of the apparatus, a drill string and by first and second seal means. The bearing chamber houses bearings and a speed reducing mechanism in a common lubricating fluid. The pressure of the lubricating fluid and external drilling fluid in a drill fluid passage are maintained substantially equal by the provision of a moveable annular piston, which is axially moveable in response to a pressure differential between the drilling fluid in the flow passage and the lubricating fluid in the bearing chamber, to equalise the pressure therebetween. This allegedly reduces the likelihood of leakage of drilling fluid into the bearing chamber. The assembly of U.S. Pat. No. 4,683,964 therefore suffers from disadvantages of the provision of the bearings together with the gear mechanism, which may lead to failure, as well as the potential for the ingress of drilling fluid causing wear.
GB 2073285 discloses a direct drive system for rotary drill bits. The system includes a drive portion, gear portion and bearing portion in a common, linked system which is not capable of being changed out on a rig floor. The system is instead assembled in a workshop as a one piece tool. The system includes an oil reservoir to provide lubricating oil to bearings and gears of the bearing and gear portions and for load compensation devices which provide a damping action in use. The bearings prevent the transmission of axial thrust forces from the turbine to the gear portion, however, there is a direct, rigid connection between the gear portion and the bearing portion. This may lead to failure as discussed above. A piston is provided which is loaded by a compression spring to exert a pressure force on the oil reservoir. However, a chamber in which the compression spring is disposed is open to drilling mud passing through the borehole returning to the surface, creating an area of hydrostatic pressure difference within a body of the system.
CA1257865 discloses improvements in the sealing arrangements for a bearing or combined bearing/gear reduction assembly. A drilling fluid is pumped down through a motor (a turbine or PDM) and flows through a chamber into a central bore of a drilling string to bypass a bearing/gear reduction chamber. An upper dynamic mechanical seal assembly is provided at the top of the chamber in a floating piston, exposed at an upper end to drilling mud. A lower dynamic mechanical seal assembly is provided at the bottom of the chamber, and together they define a lubricating fluid chamber in which bearing assemblies and a gear reduction assembly is located. Fluid in the lubricating chamber is provided at a higher pressure than the drilling mud to cause flow of lubricating fluid from the chamber, to prevent ingress of drilling fluid. This is not achieved by positively applying an over pressure on the lubricating fluid, but is dependent on fluid pressures outside the chamber. There is therefore a decreasing differential pressure as the lubrication chamber empties in service. In an alternative embodiment, a compression spring exerts a force on the floating piston to overpressure fluid in the lubricating chamber relative to the drilling fluid. The system of CA1257865 suffers from disadvantages including that the bearings are provided together with the gear mechanism, which may lead to failure, as discussed above.
PCT/EP97/06060 discloses drilling equipment, especially turbo-drills incorporating a reduction gear. The equipment comprises a turbine, a reduction gear and a spindle, the turbine including a seal and a radial thrust support. The reduction gear has input and output shafts connected through a gear, and the spindle has a body carrying a rotating shaft, a radial thrust support and a further seal. A chamber is defined by the seals and bushings of the equipment and contains lubricating oil. The equipment of PCT/EP97/06060 suffers from disadvantages including that the bearings are provided together with the gear mechanism, which may lead to failure as discussed above, and that the drilling fluid is likely to enter the sealed assembly over time.
It is an object of at least one embodiment of the present invention to obviate or mitigate at least one of the foregoing disadvantages.